It is known to truncate tubes by means of knives which penetrate through the thickness of the tube when the tube is rotated.
To truncate certain tubes and more particularly tubes made of rigid material, such as cardboard tubes, it is necessary to position within the tube a counterpart on which the knife completes its cuts.
Until the present time, the counterpart was connected mechanically to the exterior by means positioned within the tube so as to ensure the fixed positioning of the counterpart or to permit the longitudinal displacement of the counterpart.
The truncators adapted to cut unitary tubes into a plurality of sections are provided with knives and a longitudinally fixed counterpart, the latter being fitted on a fixed mandrel on which the tube is fitted.
Conversely, in a truncation apparatus for truncating a continuously formed tube, such as a spiraler for a cardboard tube, for example, it is necessary that the one or more knives and the counterpart follow the tube longitudinally in a synchronized manner in the course of manufacture, at the moment of the cut. The knives and the counterpart are then brought back to the initial position after each truncation by a means passing through the winding mandrel of the spiraler.
A mechanical linkage of the counterpart which extends towards the exterior, and which is positioned within the tube, is a true handicap and hinders certain applications.
A means which makes it possible to maintain in position and/or displace the counterpart (hereafter called the counter-knife), without the latter being connected mechanically to an exterior means has been developed. Such a truncating device is the object of parent application Ser. No. 06/924,443 (French Patent No. 2589384) which is hereby incorporated by reference. This device is especially advantageous in that the counter-knife is connected mechanically to a core of strong magnetic permeability, and a magnetic induction means is provided to create a magnetic field in the core through the tube to be truncated so as to be able to act magnetically on the longitudinal position of the core and, thus, on that of the counter-knife as a function of the longitudinal position of the knife.
According to one embodiment, the magnetic induction means comprises a winding wound in a framework around the tube to be truncated, so as to create at least substantially longitudinal lines of induction in the core. Such device is entirely satisfactory. However, the winding needs a cooling device making the assembly very heavy, especially when the tube diameter approaches 200 mm, for example.
In the above-mentioned type of apparatus, the framework is annular and has a central cylindrical part, which is made of a material with weak magnetic permeability (or susceptibility), whereas the ends of the framework comprise, conversely, elements made of material with strong magnetic permeability to direct and form the lines of induction. Thus, the lines of induction pass through the ends and go longitudinally into the core to retain it in place.
It is possible to use a device for each exterior tube diameter or for tubes having reduced diameters. In effect, when the exterior diameter of the tube becomes too small with respect to the interior diameter of the framework, the magnetic flux in the core becomes very small.